This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to radio frequency (RF) coils useful in NMR and other applications for transmitting and/or receiving RF signals.
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear-magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.o, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the equation .omega.=.UPSILON. B, in which .UPSILON. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.o plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.o, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of magnetization M by means of radio-frequency pulses through a coil connected to radio-frequency transmitting apparatus. Magnetization M rotates about the direction of the B.sub.1 field. In NMR, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.o field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane begin to realign with the B.sub.o field by a variety of physical processes. During this realignment process, the nuclear moments emit radio-frequency signals, termed the NMR signals, which are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. The same or a second RF coil may be used to receive the signals emitted from the nuclei.
An important type of RF coil is the surface coil. Rather than transmission, surface coils are typically used only for receiving NMR signals that have been excited by a separate RF transmitting coil, although they may be used for both transmission and reception. Since they are placed close to the subject area being imaged, surface coils receive a stronger signal from the spins of interest and are able to provide greater resolution than larger volume RF coils. They also provide better signal-to-noise ratios because they are sensitive to a smaller volume of tissue so that they receive a lower proportion of the noise emitted by the body.
A typical surface coil is comprised simply of a single turn of conductor which can take many shapes for different applications such as a circle, a square or a rectangle. The sensitive volume of a flat surface coil is approximately subtended by the coil circumference and is about one coil radius deep from the coil center. One drawback of a single turn surface coil is that its field homogeneity is less than optimum, no matter what its shape. A further disadvantage is related to the concentrated current flow (i.e., hot spots) which results in a large power dissipation due to electrical coupling with and losses in the body being imaged. A further problem relates to the coupling of the surface coil to the body coil during excitation or transmission by a separate transmit coil. This is usually prevented by special blocking or decoupling networks.
Accordingly, it is a principal object of the present invention to provide an RF surface coil for detecting NMR signals within a sensitivity volume.
It is another object of the invention to provide an RF surface coil having reduced power dissipation and signal loss from electrical interaction with the object being imaged.
It is yet another object of the invention to provide an RF coil having a distributed current flow and an improved signal-to-noise ratio.
It is still another object to prevent cross coupling of separate transmit and receive coils without special switched networks.
It is a further object to provide an RF surface coil conformable to various contours of an object to be studied.